Textured HTSCs on a carrier are also known as thin-film HTSCs and differ from the conventional multifilament HTSCs that are produced according to the “powder in the tube” method and from HTSCs that are produced by pre-pressing and subsequent sintering of HTSC powder especially in such a way that the superconducting cross-sectional area of the thin-film HTSC is substantially smaller. That is why it is essential for thin-film HTSCs that the HTSC layer on the carrier has the highest possible critical current density. The critical current density depends very essentially on the fact that the HTSC does not have any warpage or the like on the carrier, i.e. the HTSC layer, and thus has the most even texture, which means it is textured. The texture of an HTSC layer can be determined by means of X-ray diffractometry.
In the wet-chemical production of thin-film HTSCs, the HTSC layer must be deposited as textured as possible on the carrier. This is influenced, among other things, by the composition of the precursor solution. Typically, trifluoroacetic acid (TFA) is used in the production of the HTSC precursor solution along with at least one organic salt and/or one organic solvent and/or one organic complexing agent. If no trifluoroacetic acid (TFA) is added to the HTSC precursor solution, barium carbonate is obtained during the later heat treatment. Barium carbonate is chemically very stable; consequently, barium bonded as carbonate is no longer available for the formation of the REBa2Cu3Ox superconductor and obstructs current transport at the grain boundaries. If solvents with trifluoroacetic acid are used for the salts, barium fluoride is obtained instead of barium carbonate. Barium fluoride will react during heat treatment with water vapor to barium oxide and hydrofluoric acid. The problem is that the water vapor will at first diffuse into the HTSC precursor layer and the hydrofluoric acid needs to diffuse out of the layer. That is why only comparatively thin layers can be grown. Moreover, pores are obtained in the HTSC layer by the diffusion. It is a further disadvantage in the use of TFA that the obtained hydrofluoric acid is very poisonous and is still caustic when diluted.
It is generally understood that the problems in connection with hydrofluoric acid can be reduced by utilizing a multiple coating process in which a fluorine-free layer is applied to the carrier and, subsequently, a fluorine-containing (i.e. TFA-based) precursor solution is applied to the carrier to form the uppermost layer. The fluorine contained in the uppermost layer is sufficient, so that no relevant quantities of barium carbonate will be produced in the lower fluorine-free layer during the heat treatment.
Although methods for producing HTSC powders in which no TFA is used are generally known, it is not possible with any of these “TFA-free” methods to deposit a textured HTSC layer on a carrier.